1. Field of the Invention
The present invention relates to a braking force distribution controlling apparatus for controlling the braking force of the rear wheels of car in a predetermined relation with that of the front wheels of car during braking of car.
2. Related Background Art
When the braking operation is effected on a running car, axle weights on the front wheels and on the rear wheels differ from each other because of migration of load. Therefore, braking forces of wheels specified by slip rates and axle weights are different between the front wheels and the rear wheels. On the other hand, in order to maximize the utilization of the braking forces for stopping the car, the four wheels should be locked simultaneously. FIG. 1 is an ideal braking force distribution characteristic diagram to show distributions of front-wheel and rear-wheel braking forces when the four wheels are locked simultaneously, wherein the abscissa indicates the braking force of the front wheels while the ordinate indicates the braking force of the rear wheels. The ideal braking force distributions are different between when the car is empty and when the car is loaded. In the same figure, characteristic curve A shows the ideal braking force distribution in the empty car state and characteristic curve B the ideal braking force distribution in the normal loaded car state.
The braking force distribution controlling apparatus is intended to perform such a brake control as to make an actual braking force distribution closer to the ideal braking force distribution and performs such a control as to restrict the braking force of the rear wheels when predetermined conditions are met, in such a way that the braking force of the rear wheels is kept from exceeding the braking force of the rear wheels in the ideal braking force distribution in the loaded car state. The reason why the braking force of the rear wheels is controlled so as to be always smaller than the ideal braking force distribution is that rear-wheel braking forces higher than the ideal braking force distribution will degrade driving stability.
According to Japanese Laid-open Patent Applications No. Hei 6-144176 and No. Hei 6-144178, etc., the braking force distribution control is started to restrict the braking force of the rear wheels when an estimated body deceleration becomes a predetermined deceleration or more and when a velocity difference between a wheel velocity of front wheel and a wheel velocity of rear wheel becomes a predetermined value or more. This shows the characteristic like broken line C as shown in FIG. 1, for example.
In the case of the conventional braking force distribution controlling apparatus as described above, however, when so-called irregular size tires other than standard tires are mounted, errors are produced in detected values of wheel velocity, so that the velocity difference is not obtained accurately between the wheel velocity of front wheel and the wheel velocity of rear wheel. Especially, when the irregular size tires are mounted as only either the front wheels or the rear wheels, inaccuracy of the velocity difference is enhanced furthermore.
Owing to it, there are such cases that the braking force distribution control is not started even though it is necessary while satisfying the condition that the actual body deceleration and the actual front-rear wheel velocity difference each are equal to or greater than the respective predetermined values.